It is often necessary or desired to provide a coating of a particular substrate. For example, in the micro-electronics industry it is often desired to coat a substrate used in the manufacture of integrated circuits for further processing. Often it is required that such coatings be applied in a very thin coat, such as a thin coat of photoresist used in masking and etching a silicon substrate for manufacturing integrated circuits, which is uniform across the entire surface of the substrate. However, as the coating is so thin, very minute variances in its thickness may not be acceptable.
Accordingly, the prior art has relied upon various methods for providing a continuous, uniform, thin coating of a substrate. However, in the past these methods have been inefficient and, therefore, prone to waste.
For example, a commonly relied upon prior art method of coating a substrate is spin coating. Here a coating material, typically suspended in a solvent based fluid, is deposited in a pool on the substrate to be coated, generally at or near the center of the surface to be coated. Thereafter, the substrate itself is rotated at a high speed about an axis normal to the surface to be coated. Centrifugal forces created by the rotation of the substrate cause the pool of material to migrate toward the edges of the substrate. Accordingly, rotating the substrate for a sufficient length of time at a proper speed will result in a substantially uniform coating having a desired thickness, where a sufficiently ductile coating material is present.
However, the spin method of coating the substrate necessarily results in an amount of coating material being expelled from the surface to be coated. In practice, the expelled portion of coating material may be as great as 90-95% of the material initially deposited in the pool on the substrate. Typically this material which is expelled from the surface is lost as there are often very stringent purity requirements and/or the solvents suspending the material being quick to evaporate making their recycling difficult or impossible. Moreover, spin coating is generally not completely effective in evenly distributing a very viscous coating material.
These coating materials are generally very expensive and therefore the waste that occurs in coating the substrate can be an important consideration. Accordingly, although providing a reliable method for achieving a uniform coating of a substrate, the prior art spin methods introduce an undesired level of waste. Moreover, the amount of waste increases as the size of the substrate increases causing such inefficiencies to be increasingly unpalatable as the industry moves to larger and larger substrates, e.g., 3.5 generation LCD technology and 12 inch silicon wafers and still larger flat panel displays (FPDs).
It should also be appreciated that the excess material discharged from the above mentioned spin technique presents, at a minimum, a requirement for the subsequent handling and cleanup of this substantial amount of unused material. Because of material purity requirements, this discharged material must often be disposed of However, often this material, and/or its solvent carrier, are hazardous materials, and must therefore be handled with extreme care as well as being disposed of in accordance with stringent guidelines. Likewise, often the solvents utilized in cleanup of such discharged material are hazardous, thus compelling their restricted use.
Additionally, the prior art spin methods of coating the substrate can result in the outer edges and/or the back surface of the substrate also being coated by the material. This can be undesired as subsequent handling of the substrate, having its edges coated, may result in the chipping and peeling of the coating on these edges which effects may propagate onto the surface for which a uniform coating is desired. Moreover, coating of the back edges and/or back surface may also result in the contamination of the surface desired to be coated.
Furthermore, the solvents carrying the desired coating materials in suspension may be highly unstable and, therefore, prone to rapid dissipation, such as through evaporation. Accordingly, uneven coating may result in the aforementioned spin technique where, for example, an appreciable time between depositing the pool of material for spinning, or where the substrate surface to be coated is large.
In spite of the disadvantages of the spin coating method, it has been employed when coating silicon wafers, flat panel displays, and any surfaces which are particularly sensitive to the presence of contaminants. The spin coating equipment, and in particular, the fluid handling surfaces of the spin coating equipment can be constructed out of non-metallic material thereby reducing contamination of the coating fluid arising from contact between the coating fluid and metal surfaces. Spin coating for such surfaces has therefore been acceptable in spite of the disadvantages of the method listed above.
In a typical prior art embodiment, a TEFLON# (available from E.I. DuPont De Nemours and Company) or other non-metallic tube dispenses coating material onto a substrate being spun or otherwise moved underneath the tube. Such an embodiment does not require delicate machining or that precise tolerances be maintained, thereby permitting the production of non-metallic spin coating fluid dispensing to be accomplished in a straightforward manner.
Extrusion dies may be used for the application of layers of liquid polymers to various surfaces or substrates as shown and described in the incorporated patent application Ser. No. 09/148,463. The liquid polymer is typically dispensed through the extrusion die at a precise rate as the die is moved relative to the substrate at a fixed distance from the substrate surface. Extrusion dies are fabricated from various types of steel alloys since these materials offer the following properties which are generally required for the dies: dimensional stability, high hardness, machinability, capability of achieving a high degree of flatness and surface finish, compatibility with the process polymers, and high temperature capability (in some cases). When using extrusion dies for an application that requires very low levels of contamination, such as the fabrication of microelectronics or flat panel displays, then a corrosion resistant steel alloy (commonly referred to as "Stainless Steel") may be used.
However, for the fabrication of many microelectronic parts and of some flat panel displays, even corrosion resistant alloys are not acceptable, due to general contamination/cleanliness concerns and more particularly to ionic contamination which occurs when the process fluid leaches some of the metal ions out of the steel thereby contaminating the coating fluid passing through the head. A further source of contamination can result from actual removal of material from the extrusion head as a result of mechanical abrasion of the coating fluid motion against the extrusion head surfaces.
For these special applications and perhaps others, it is desirable to have a material and fabrication process that will have the mechanical properties required for extrusion dies but without metallic components that contact the process material.
When using metal extrusion heads, prior art mechanisms, such as shown and described in the incorporated patent application Ser. No. 09/148,463, have typically used metal shims to accurately space or separate parts of a dispensing or extrusion head in order to create an extrusion slot and thereby achieve desired flow characteristics. A preferred metal for the shim has been stainless steel. A problem with this approach is that the shim is typically made of different metal than the metal parts being separated, and a galvanic potential is generated between the main metal components and the shim. The combination of this galvanic potential in the presence of fluid flow results in corrosion which can worsen the emission of contaminants into dispensed fluid. In this case, the dispensed fluid could carry those contaminants arising from reaction between the fluid and the metal surfaces, those arising from abrasion or mechanical removal of material by the fluid, those arising from leaching of alloy material from the metal surfaces into the fluid, and finally, those resulting from the galvanic induced chemical reaction between the extrusion head metal and the shim. Further, certain process fluids contain chlorine which may induce corrosion between the fluid and a stainless steel shim, particularly at the interface between the head and the shim.
Therefore, where metal extrusion heads and/or chlorine-containing coating fluids are used, there is a need for a shim which will avoid the problem of galvanic potential induced corrosion and the fluid contamination resulting therefrom, and chemical activity between the shim and the coating fluids.
There is a further need in the art for dispensing and/or extrusion equipment which is sufficiently non-contaminating so as to be usable for coating silicon wafers and flat panel displays.
There is a further need for a non-contaminating extrusion head which is small, light, hard, durable, resistant to warpage, and which can be machined to precise tolerances.